T
tiler robbo
If bath is level (and it should be) and full tile from bath works everywhere else than I would tile off the bath personally. Why cut? It's like full tile off a kitchen worktop - it looks better.
Try middle of tile and/or grout line in center of window and see what cuts it leaves, if neither of them work then tweak the best of the two left or right.
Will the bath act as a batton & stop the tiles slipping ok? If I do it this way, it will leave me a tile & a half to fix under the batton area, I'm assuming tiling down from here to the floor will be ok?